1. Field of the Invention
Aspects of the present invention relate to an electrolyte for a high voltage lithium rechargeable battery and a high voltage lithium rechargeable battery employing the electrolyte; aspects of the present invention relate more particularly to an electrolyte for a high voltage lithium rechargeable battery which includes a halogenated biphenyl and dihalogenated toluene used as additives where the additives have an oxidation reduction potential relative to lithium of 4.6 to 5.0 V, as well as a high voltage lithium rechargeable battery that has overcharge stability.
2. Description of the Related Art
Recently, as portable electronics, such as camcorders, cellular phones and laptop computers have become widespread in daily life and as electronic appliances have been miniaturized and lightened due to the rapid development of technology in the electronics, network and computer industries, consumers require light, durable and reliable batteries more and more. In particular, lithium rechargeable batteries have an energy density per unit weight three times as much as those of conventional lead-acid storage, nickel-cadmium, nickel-metal hydride and nickel-zinc batteries. Furthermore, a lithium rechargeable battery can be recharged rapidly. Consequently, lithium rechargeable batteries have been studied and developed throughout the world.
In the lithium rechargeable battery, a lithium metal oxide is used as a positive electrode active material, and a lithium metal, lithium alloy and carbon or carbon composite are used as a negative electrode active material. A lithium rechargeable battery is classified as a lithium ion, lithium ion polymer or lithium polymer battery according to the type of the electrolyte used therein, and it is classified as a cylinder type, polygonal type or coin type battery according to the shape of the battery.
In a lithium rechargeable battery, the most important problem is ensuring the physical security of the battery during operation. In particular, a lithium ion rechargeable battery reaches an overcharged state when it is charged over the designed level, for example due to a malfunction in the charging control circuit. Then lithium ions are pulled out of the positive electrode and moved to the negative electrode, where lithium ions are both occluded in the negative electrode and precipitated on the surface of the negative electrode. In this state, if the battery continues to be charged, then the internal resistance of the battery increases because of the movement, occlusion and precipitation of the lithium ions. This behavior of the lithium ions is an exothermic reaction, and in the worst case thermal runaway occurs.
To prevent or control the overcharge problem or thermal runaway, a temperature sensing current cut-off switch, for example a PTC (positive temperature coefficient thermistor) to cut off the current, or a means which reduces the charging current upon detection of an internal pressure change in the battery, is generally used. However, the use of a current cut-off mechanism is expensive, and installation of such a mechanical apparatus inside the battery is difficult because of the need for miniaturization of the battery.
For resolving these problems, electrolytes including biphenyls and alkyl-benzenes are known from publication patents such as JP 1997-17184, JP 2000-58116, JP 2001-15155 to promote thermal stability as a function of the charging state of the battery. Furthermore, in Japanese publication patent JP 1997-50822 and JP2000-58117, the internal temperature rise can be prevented by adding into the battery an aromatic compound with a methoxy group and halogen group, biphenyl or thiophene, or an aromatic ether compound which polymerize in the overcharged state.
However, when biphenyl (and other additives) are used, at a typical operating voltage, the biphenyl is gradually decomposed during charging/discharging of the battery when relatively high voltage is locally generated, or decomposed when the battery is discharged at high temperatures for long periods, such that overcharge safety cannot be guaranteed after 300 charge/discharge cycles. Additionally, the additives for conventional overcharge protection used in a battery operating at 4.2 V can not ensure the overcharge safety of a high voltage battery (4.4 V system) and can cause deterioration of the reliability of a 4.4 V system.